Method and apparatus for operation, protection, and restoration of heterogeneous optical communication networks

ABSTRACT

Techniques for providing normal operation and service restoration capability in the event of failure of terminal equipment or transmission media in a heterogeneous network, such as a hybrid network containing single- and multi-wavelength lightwave communications systems. An optical switching node (OSN) is placed at each node in the ring network to provide the required connections between various fibers and terminal equipment, but having switch states that allow signals on the protection fibers to bypass the terminal equipment at that node. Ring-switched signals propagate around the ring on protection fibers without encountering the terminal equipment at the intervening nodes. To the extent that the protection fiber links between any given pair of nodes are incapable of supporting all the relevant communication regimes, such links are modified to provide such support.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.09/714,378, filed Nov. 15, 2000, now U.S. Pat. No. 6,839,514, which is acontinuation of U.S. patent application Ser. No. 09/408,002, now U.S.Pat. No. 6,331,906, which was filed on Sept. 29, 1999 and claimspriority from the following U.S. Applications, the disclosures of which,including all attached documents and appendices, are incorporated intheir entirety for all purposes:

-   -   Application Ser. No. 60/038,149, filed Feb. 10, 1997, of Rohit        Sharma and Larry R. McAdams, entitled “METHOD AND APPARATUS FOR        SIMULTANEOUS OPERATION OF SINGLE AND MULTIPLE WAVELENGTH        LIGHTWAVE COMMUNICATION NETWORKS.”    -   Application Ser. No. 09/019,347, now U.S. Pat. No. 5,986,783,        filed Feb. 5, 1998, of Rohit Sharma and Larry R. McAdams,        entitled “METHOD AND APPARATUS FOR OPERATION, PROTECTION, AND        RESTORATION OF HETEROGENEOUS OPTICAL COMMUNICATION NETWORKS.”    -   Application Ser. No. 09/020,954, now U.S. Pat. No. 6,046,833,        filed Feb. 9, 1998, of Rohit Sharma and Larry R. McAdams,        entitled “METHOD AND APPARATUS FOR OPERATION, PROTECTION, AND        RESTORATION OF HETEROGENEOUS OPTICAL COMMUNICATION NETWORKS.”

COPYRIGHT NOTICE

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

The following 6 microfiche appendices comprising 342 frames on 4 sheetsof microfiche were filed as part of U.S. pat. application Ser. No09/408,002, now U.S. Pat. No. 6,331,906, filed Sep. 29, 1999, of RohitSharma and Larry R. McAdams, entitled “METHOD AND APPARATUS FOROPERATION, PROTECTION AND RESTORATION OF HETEROGENEOUS OPTICALCOMMUNICATION NETWORKS,” and are incorporated by reference in theirentirety for all purposes:

-   -   Appendix 1—96 pages of source code;    -   Appendix 2—(51pages) Span Switch Restoration States (file        Restoration₁₃States₁₃Span);    -   Appendix 3—(62 pages) Ring Switch Restoration States (file        Restoration ₁₃States₁₃Ring)    -   Appendix 4—(58 pages) P-Transit Switch Restoration States (file        Restoration_States_Transit)    -   Appendix 5—(52 pages) Adjacent Node Action Request Table (file        ANAR₁₃Table); and    -   Appendix 6₁₃ ₍44pages)“OSN Operation: SONET and WDM Network        Elements.”

BACKGROUND OF THE INVENTION

The invention relates generally to optics and communications, and morespecifically to optical fiber based networks, techniques for restorationof network services in the event of a failed fiber link (e.g., a breakin a fiber or a failure of an active element such as a fiber amplifier)and the use of optical switching to effect such restoration.

Photonic transmission, amplification, and switching techniques provideflexible means of provisioning, configuring, and managing the modernhigh capacity telecommunication networks. The physical layer in thenetwork, which includes the transmission equipment and the fiber layerused for signal transport, is required to be capable of reconfigurationof facilities in order to support dynamic routing of traffic. While slowreconfiguration of the order of minutes or more may be sufficient forrearranging traffic capacity in response to change in demand patternsacross the network, rapid reconfiguration (perhaps 50 ms or less) isrequired for restoring services in the case of transmission equipment orfiber cable facility failures. Fast restoration is also critical toprevent escalation of the effects of a single point of failure where theaffected services (voice and data) attempt to reconnect immediatelyfollowing the disruption of services and may lead to overloading offacilities adjacent or connected to the point of original failure.

In addition to the critical need for fast restoration, the capacity thatneeds to be re-routed has increased rapidly with the continuing increasein data rates for optical transmission and the introduction ofmulti-wavelength channels on a single fiber. For example, the rapidgrowth in traffic capacities required for long haul telecommunicationsnetworks has accelerated the introduction of new technologies fortransmission and multiplexing. Transmission links up to bit rates of 10Gbps (OC-192) are in commercial service and new developments inmulti-wavelength component technologies are resulting in increasedcommercial availability of 4-, 8-, 16-, 32-, and 40-channel WDM(wavelength division multiplex) links (at 2.5 Gbps per wavelength ormore).

Transmission of such high data rates over single fibers also results inmaking the network more vulnerable to failures of larger magnitude. Forexample, a single fiber link failure can disrupt approximately 130,000voice channels (DS0) when the fiber link is operating at 10 Gbps on asingle-wavelength or at 2.5 Gbps on each of four wavelengths.Consequently, redundant facilities provisioned for dynamic restorationof service also need to provide a similar magnitude of capacity on thelinks used as backup or spare links for ensuring network survivability.

Therefore, routing techniques used for network restoration must providesolutions that are compatible with the twofold requirement of fastswitching and high capacity.

International and North American standard bodies have defined variousSynchronous Optical Network (SONET) configurations for operation oflightwave networks. “Self-healing ring” configurations allow for rapidrestoration of services in the event of a failure of fiber transmissionmedia. In a four-fiber self-healing ring network, each node is connectedto its adjacent nodes through two pairs of fibers (carrying signals inopposite directions). One fiber in each such pair is called the“working” fiber; the other fiber is termed the “protection” fiber andmay be used when the working fiber facility fails. Each node includesadd-drop multiplexer (ADM) terminal equipment that originates andterminates signals traversing the various links in the ring.

When a failure of any working fiber link between any two nodes occurs,the ADM terminal equipment on either side of the failure carries out therequired re-routing of signals over protection fibers. Such re-routingof signals to restore all services is referred to as “restoration” ofservices. If an outgoing working fiber link fails, but the correspondingprotection fiber link is intact, the signals intended for the failedworking fiber will be diverted to the intact corresponding protectionfiber in what is referred to as span switching. In this context,reference to the corresponding protection fiber means the protectionfiber coupled between the same two nodes and for use in the samedirection (to or from the other node).

If the working and protection links fail, the signals intended for thefailed working fiber will be directed to the outgoing protection fiberin the other direction around the ring, being passed from one node tothe next, in what is referred to as ring switching.

However, some of these restoration schemes (ring switching) break downin what will be referred to as heterogeneous networks. A heterogeneousring network is one where different links differ in some materialrespect such as signal-carrying capacity (bandwidth), number ofwavelength channels, modulation scheme, format, or protocol. Forexample, certain high-traffic links may have been upgraded to provideincreased bandwidth, by increasing the bit rate of signals on a givenwavelength channel, by providing additional WDM terminal equipment tosupport additional wavelength channels, or both.

Thus, for a variety of reasons, the network may have a link, withterminal equipment at each end, where the signals on that link are alienor unsupported on one or more other links. Since at least some link inthe opposite direction will not support the signals that normally travelon the failed link, ring switching is not possible. A particular type ofheterogeneous network, namely one containing single- andmulti-wavelength lightwave communication links, is sometimes referred toas a hybrid network.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for providingnormal operation and service restoration capability in the event offailure of terminal equipment or transmission media in a heterogeneousnetwork, such as a hybrid network containing single- andmulti-wavelength lightwave communications systems.

In general, this is accomplished by allowing ring-switched signals topropagate around the ring without encountering the terminal equipment atthe intervening nodes. To the extent that the protection fiber linksbetween any given pair of nodes are incapable of supporting all therelevant communication regimes, such links are modified to provide suchsupport.

In specific embodiments, an optical switching node (OSN) is placed ateach node in the ring network to provide the required connectionsbetween various fibers and terminal equipment, but having switch statesthat allow signals on the protection fibers to bypass the terminalequipment at that node. In the context of a hybrid network where onlysome nodes have WDM terminal equipment, normal operation and restorationof multi-wavelength signals become possible without disturbing thesingle-wavelength SONET operation of that ring.

The steps, if needed, to upgrade the protection links depend on thenature of the network heterogeneity, but are generally relativelyinexpensive. For example, upgrading the protection links to supportmulti-wavelength or higher bit-rate operation often entails no more thanthe addition of appropriate optical amplifiers. Routing or re-routingfor restoration of the high-bandwidth (e.g., multi-wavelength) traffictakes place through the OSNs. It is not necessary to provide specialterminal equipment capable of terminating the high-bandwidth signals atthe nodes that are not normally required to handle such signals, sincethose nodes are bypassed due to the operation of the OSNs.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show typical prior art ring and mesh network topologies;FIG. 2A is a schematic of a prior art four-fiber bidirectional lineswitch ring (BLSR);

FIG. 2B is a detail view of an add-drop multiplexer (ADM) at one of thenodes;

FIG. 3A shows span restoration in a SONET ring;

FIG. 3B shows ring restoration in a SONET ring;

FIG. 4 shows a typical hybrid configuration with a number ofsingle-wavelength rings sharing a multi-wavelength link;

FIG. 5 shows a prior art restoration scheme using optical switches;

FIG. 6 is a schematic of a four-fiber hybrid BLSR as upgraded accordingto an embodiment of the invention;

FIGS. 7A and 7B are schematic views of an optical switching node (OSN)according to an embodiment of the invention;

FIGS. 8A and 8B are schematic views showing a number of possibleswitching paths within the OSN;

FIGS. 9A–9O show 15 span switching states for the OSN;

FIGS. 10A–10T show 20 ring switching states for the OSN;

FIGS. 11A–11R show 18 protection fiber transit (P-transit) states forthe OSN;

FIG. 12 is a schematic showing a particular implementation of theoptical switches in the OSN;

FIG. 13 shows additional details of the OSN, including control logic andadditional elements to implement a restoration supervisory channel;

FIG. 14 illustrates full ring switching by the OSNs in response to afailure of all four fiber links between a pair of nodes having WDMequipment;

FIGS. 15A–15C, taken together, form a flowchart of the OSN software in aspecific embodiment

FIGS. 16A–16C show the use of OSN sub-modules to achieve additionalfunctionality;

FIGS. 17A and 17B show implementations of bidirectional supervisorychannels;

FIG. 18 shows an implementation of bidirectional WDM network datatransmission; and

FIGS. 19A–19C show the OSN deployed in networks having mixed types ofterminal equipment.

DESCRIPTION OF SPECIFIC EMBODIMENTS

1.0 Technological Overview

1.1 Network Layers

Discussions of network communications are often with reference to anetwork layer model, such as the International Standards Organization(ISO) Open Systems Interconnection (OSI) reference model. The OSIlayers, from the highest to the lowest, include the application layer,the presentation layer, the session layer, the transport layer, thenetwork layer, the data link layer, and the physical layer.

The application layer supports semantic exchanges between applicationsand provides access to the lower functions and services. Thepresentation layer deals with representing data to the end user orapplication. The session layer organizes and structures the interactionbetween applications and devices. The transport layer is responsible fortransparent and reliable transfer of data without regard to the natureand attributes of the transfer medium. The network layer establishescommunication between networks and is concerned with addressing,routing, and relaying information. The data link layer providesfunctions and protocols for transferring data between network resourcesand detecting errors in the physical layer. The physical layer, inaddition to defining the actual mechanicals electrical, or opticalcharacteristics of the communication medium, also defines the functionaland procedural standards for the physical transmission of data over thecommunications medium.

The physical layer is itself often considered to include a number ofsublayers including, from top to bottom, the line terminal equipment(LTE) layer, the photonic connectivity layer, and the fiberinfrastructure.

1.2 Fiber Technologies

Optical fiber links used in telecommunications are eithersingle-wavelength or multi-wavelength. In a fiberoptic communicationsnetwork, an electrical signal is converted to an optical signal,typically by modulating a laser diode emitting light at a wavelengthsuited for propagation along the fiber. The modulated light is injectedinto the fiber and detected by a fiberoptic receiver that includes aphotodiode or other opto-electronic device to retrieve a signalcorresponding to the original electrical signal. It is possible totransmit many signals on an optical fiber at the same time using atechnique known as wavelength division multiplexing (WDM). Light at anumber of different wavelengths is injected into a single fiber using awavelength multiplexer, and the light at the individual wavelengths areseparated at the other end using a wavelength demultiplexer.

Wavelength multiplexers and demultiplexers are often referred togenerically as WDM couplers. It is also possible to use WDM couplers toestablish bidirectional operation on a single fiber, and WDM couplersoptimized for such use are often referred to as bidirectional WDMcouplers. Some other optical elements such as isolators and circulatorsare often used in conjunction with WDM couplers to establishbi-directional communications over a single fiber with reducedcross-talk, back-reflection, etc. Although these elements are notessential in theory, they are useful in practice.

A given optical fiber that supports transmission at a given wavelengthis likely to support transmission at a number of closely spacedwavelengths. However, it is typically the case that optical amplifiersare disposed at various locations in the path, say every 30 km, and atypical single-wavelength fiber link is likely to have amplifiers thatonly operate correctly for the single wavelength that is beingtransmitted on the fiber. A different optical amplifier may be needed tosupport multi-wavelength operation.

Prior to the development of erbium-doped fiber amplifiers, it wasnecessary to interpose regenerators (sometimes referred to as repeaters)in order to maintain the signal. A regenerator would convert the opticalsignal to an electrical signal, amplify the electric signal, and thenreconvert the amplified electric signal to an optical signal. Theregenerator may also reshape or otherwise condition the electricalsignal and resynchronize the electrical signal to a master network clock(if available) before reconverting it to an optical signal. Commonwavelengths for use in fiberoptic transmission include wavelengths inthe neighborhoods of 1310 nm and 1550 nm. However, the erbium amplifiersoperate only in the 1550-nm range, and so as a practical matter, 1550 nmremains the wavelength of choice. In a typical multi-wavelengthenvironment, the wavelengths are spaced by 0.8 nm (corresponding to 100GHz at 1550 nm) or 1.6 nm, and are typically in the range of 1530–1570nm. It is noted that a regenerator for multi-wavelength fiber wouldrequire that each wavelength be separately regenerated, thus requiring aWDM demultiplexer for sending each wavelength on the incoming fiber to aseparate regenerator and a WDM multiplexer to recombine the regeneratedoptical signals onto the outgoing fiber.

There are two classes of optical fiber, referred to as single-mode andmulti-mode. While multi-mode fiber is relatively inexpensive, it istypically used only for short data communications applications (say 50meters or less). This is because the different modes of propagation havedifferent transit times along the fiber, so dispersion becomes asignificant factor over long distances.

1.3 SONET Restoration

In the discussion that follows, the specific type of network is asynchronous optical network (SONET), which uses time divisionmultiplexing (TDM) wherein multiple channels are given different timeslots within a frame. Each node includes an add-drop multiplexer (ADM)that interfaces the fibers to the electronic devices that are tocommunicate with each other over the network. A SONET network providesreliable transport from point to point and has the capability ofproviding restoration. However, the invention finds applicability withother types of terminal equipment, such as devices for routing ATM, IP,or other types of packet or synchronous data.

The SONET ADM provides two broad functions. The first function isextracting information in one of the time slots from the incomingworking fibers and outputting information into that time slot fortransmission (along with the information in the other time slots) on thefiber that continues in the same direction. The second function isperforming electrical switching to reroute information onto theprotection fibers in the event of a failure in one or more of the fiberlinks. In this application, the term “link” will be used to refer to acommunication path between two nodes. The term “span” is sometimes usedto refer the same thing.

FIG. 1A shows a typical prior art ring network topology. FIG. 1A shows aring network in which a plurality of nodes 20 are interconnected byfibers 25. FIG. 1A shows a bidirectional ring wherein each node can sendand receive signals to and from adjacent nodes on the ring. The nodesare designated 20 a, 20 b, 20 c, etc., and are denoted as having ADMs.The nomenclature regarding the fibers is that a fiber for propagatingsignals from a given node to an adjacent node is provided with thesuffix comprising the letter suffix of the originating node and thereceiving node. For example, node 20 a sends signals to node 20 b onfiber 25 ab and receives signals from node 20 b on fiber 25 ba.

While from the point of view of the ring, the directions are referred toas clockwise and counterclockwise, from a node's point of view, the twodirections are arbitrarily designated upstream and downstream, or westand east.

FIG. 1B shows a typical prior art mesh network wherein a plurality ofnodes 30 send and receive signals to and from other nodes in the networkvia fibers 35. In this case, at least some nodes are capable of sendingand receiving signals directly to and from more than a pair of adjacentnodes. In the particular example shown, there are four nodes in thenetwork, and each has a direct connection to the other three nodes.

FIG. 2A is a schematic of a prior art four-fiber bidirectional lineswitch ring (BLSR). FIG. 2A differs from FIG. 1A in that each fiber inFIG. 1A has a counterpart in FIG. 2A of a pair of fibers, called theworking and protection fibers. The ADMs are designated 50 a, 50 b, 50 c,and 50 d, and the four-fiber links between adjacent nodes are designated55.

FIG. 2B shows additional details of one of the ADMs, say ADM 50 a. It isconvenient to designate each fiber in the network according to anexpanded version of the numbering scheme from FIG. 1A, where each fiberhas a further suffix w or p designating whether it was a working fiberor a protection fiber. The ADM has 8 fiber ports, two input from each oftwo directions and two output towards each of those two directions,designated west and east. Each fiber pair as noted above includes aworking and protection fiber. Each input port communicates with anoptical receiver 57 that converts the modulated optical signal to acorresponding electrical signal. Each output port communicates with anoptical transmitter 58 that converts the electrical signal to acorresponding optical signal.

ADM 50 a includes west-to-east demultiplex-multiplex units 60 andeast-to-west demultiplex-multiplex units 62. The demultiplex portion ofeach demultiplex-multiplex unit separates the incoming signals in thedifferent time slots and conditions them; the multiplex portion of eachdemultiplex-multiplex unit places (combines) the individual conditionedsignals into their respective time slots for retransmission. Thedemultiplex-multiplex units associated with the working fiber portsremove data intended for that node (shown schematically as an arrowpointing downwardly away from the unit) from one or more of the timeslots and insert data intended for the next node (shown as an arrowpointing downwardly into the unit) into one or more of the now vacanttime slots. The demultiplex-multiplex units associated with theprotection fiber ports need not provide this add-drop functionality,although typical implementations provide the add-drop functionality forall the demultiplex-multiplex units in order to provide the maximumversatility.

ADM 50 a further includes provision for electrical switching so thateach demultiplex-multiplex unit can receive its data from any of thereceivers and output its data to any of the transmitters. ADM 50 a alsoincludes provision to pass signals from a protection receiver to thecorresponding protection transmitter without change.

FIG. 3A shows what is referred to as span restoration (or spanswitching). This is when a working fiber breaks or some other equipment(e.g., an amplifier) on the working link fails. In the specific example,ADMs 50 a and 50 b are connected by working and protection fibers 55 abwand 55 abp for communications from ADM 50 a to ADM 50 b, and further byworking and protection fibers 55 baw and 55 bap for communications fromADM 50 b to 50 a. In the specific example, the link defined by workingfiber 55 abw has failed, making normal communications from ADM 50 a to50 b impossible. The drawing is simplified in that the fiber porttransmitters and receivers and some of the demultiplex-multiplex unitsare not shown.

SONET restoration provides for electrically switching the signals thatwould otherwise have been directed to the transmitter for working fiber55 abw so that they are directed to the transmitter for protection fiber55 abp. Furthermore, the electrical switching at ADM 50 b recognizesthat the signals otherwise expected to be received from the receiver forworking fiber 55 abw are instead to be obtained from the receiver forprotection fiber 55 abp, and performs appropriate routing so that thesignals that are to be transferred to ADM 50 c (not shown) arecommunicated to the transmitter for working fiber 55 bcw.

FIG. 3B shows what is referred to as ring restoration (or ringswitching), which is required when a working fiber link and itscorresponding protection fiber link fail (the figure shows the moreextreme case where both working fibers and both protection fibers fail).In this case, signals that were to be communicated from ADM 50 a to ADM50 b on working fiber 55 abw are directed to be output in the oppositedirection on protection fiber 55 adp. The other ADMs in the ring, exceptfor ADM 50 b, receive the incoming data on the protection fiber andforward it to the next node unchanged. Thus, the signals that wereotherwise to be received by ADM 50 b on working fiber 55 abw arereceived on protection fiber 55 cbp. Similarly, the signals that wereintended to be sent from ADM 50 b to ADM 50 a on working fiber 55 baware rerouted to protection fiber 55 bcp and propagate around the ring inthe opposite direction, being received by ADM 50 a on protection fiber55 dap.

It is assumed in the above discussion that all of the fiber links aresingle-wavelength. The scheme could be implemented with all the fiberlinks being multi-wavelength if every node had WDM equipment formultiplexing and demultiplexing the individual wavelengths and if theSONET electrical terminal equipment (ADMs) were replicated for eachwavelength.

FIG. 4 shows what is referred to as a hybrid network with a plurality ofsingle-wavelength four-fiber BLSRs 70, 72, and 75, but having a sharedmulti-wavelength link 80 between a pair of nodes 82 a and 82 b that arecommon to the three rings. It is possible to implement this since themulti-wavelength link can provide transmission capacity equal to aplurality of single-wavelength links between the nodes. Conceptually,multi-wavelength link 80 can just be considered (in the particularexample) as performing the function of 3 single-wavelength links, eachdedicated to its particular ring. However, for the example shown, theoriginal reason for outfitting link 80 to a multi-wavelengthconfiguration is that the traffic between nodes 82 a and 82 b may beexceptionally heavy, and the larger number of wavelengths may besupported on that link.

In the event of a failure in multi-wavelength link 80, communicationsbetween ADM 82 a and ADM 82 b can be restored if there are enoughsingle-wavelength rings, such as rings 70, 72, and 75 whose protectionfibers could be used to reroute each wavelength channel on a separatering as discussed above. To the extent that the number of differentwavelengths on link 80 exceeds the number of protection rings, networkservices between ADMs 82 a and 82 b will be severely disrupted.

As a matter of terminology, restoration and protection are different,but the term restoration is typically used generically to refer to both.Protection refers to the fact that resources have been committed tocarrying the data (e.g., dedicated alternate paths or bandwidth and amechanism for switching). Restoration, when used in the specific sense,refers to the ability to actively search for capacity in event of afailure, which is relevant in mesh networks. It may be necessary to slowdown or disrupt other communication to find the extra path.

1.4 Optical Switching

FIG. 5 shows a prior art configuration using optical switching forrestoration. The figure shows four ADMs 90 a–90 d and working andprotection rings. The working ring includes fiber links 92 ab, 92 bc, 92cd, and 92 da; the protection ring includes protection fiber links 95ad, 95 bc, 95 cd, and 95 da. The figure only shows one-half of thenetwork. In a bidirectional network, additional working and protectionfiber rings would be present, and additional switches would be provided.

Each ADM has associated optical switches under control of the ADMs. Forexample, ADM 90 a has 1×2 switches 100 a and 102 a and a 2×2 switch 105a, and ADM 90 b has 1×2 switches 100 b and 102 b and a 2×2 switch 105 b.The 1×2 optical switches have what are referred to as primary andsecondary states. The 2×2 optical switches have what are referred to ascross and bar states. In the network's normal mode of operation, the 1×2switches are set to their primary states so that the working ring iscoupled to the ADMs in the normal way. For restoration, as for exampledealing with a break in the working ring between ADMs 90 a and 90 b,switch 102 a would be switched to its secondary state divert light thatwould otherwise be directed to working link 92 ab to 2×2 switch 105 andonto the protection ring.

Assuming a failure in working link 92 ab, 1×2 switch 102 would be set toits secondary state so as to divert the light, which would normally besent on link 92 ab, to 2×2 switch 105 a, which would be set to itscross-state to divert the light onto protection link 95 da. Theremaining 2×2 switches would be set to their bar states in order to passthe light to ADM 90b's associated 2×2 switch 105 b, which would be setto its cross-state in order to communicate the light to ADM 90 b through1×2 switch 100 b, which would be set to its secondary state. The fiberlinks traversed by the light are marked with large black dots.

2.0 Network Retrofitting and Optical Switching Node (OSN) Overview

2.1 Retrofit

FIG. 6 shows how a hybrid ring network 110 can be retrofitted andupgraded to support multi-wavelength restoration. For ease ofdescription, the network configuration and fiber nomenclature of FIGS.2A and 2B are used with corresponding elements having the same referencenumbers. Primed reference numbers are used to designate multi-wavelengthcapability, and fibers that are multi-wavelength capable are drawn inheavy lines. In the particular example, a representative ring networkhaving ADMs 50 a, 50 b, 50 c, and 50 d has been upgraded so that workingfibers 55 abw′ and 55 baw′ and protection fibers 55 abp′ and 55 bap′ aremulti-wavelength capable for bidirectional multi-wavelengthcommunications on the link between ADM 50 a and 50 b. Thus this ring canshare this link with other rings, as indicated in the figure.

While the figure is drawn with separate unidirectional fibers for eachbidirectional working link and for each bidirectional protection link,the invention can be implemented in an environment where one or more ofthe bidirectional links consists of a single fiber carrying signals atone set of wavelengths in one direction and signals at a different setof wavelengths in the other direction. This would require WDM equipmentat each end to separate the optical paths for the two sets ofwavelengths. It is also possible to implement the invention in anenvironment where the working and protection capacity (shown as separateworking and protection fibers) is provided on a single fiber having asufficient number of wavelength channels to replicate the necessarybandwidth.

According to embodiments of the present invention, such restorability isprovided by the interposition of optical switching nodes (OSNs) 120 a,120 b, 120 c, and 120 d between the ADMs and the fiber rings, andfurther by retrofitting the protection fibers in the other links so thatthey are multi-wavelength capable. For example, the protection fibers inthe link between OSN 120 a and OSN 120 d, designated 55 adp′ and 55dap′, are multi-wavelength capable. As noted above, thesingle-wavelength protection fibers are generally capable of supportingmulti-wavelength operation, but it may be necessary to change theamplifiers, if present in the link, to amplifiers having a wider gainband to support multi-wavelength operation.

Each OSN includes optical switch elements and control electronics forcontrolling the optical switch elements. In a specific embodiment, thecontrol electronics is also responsible for determining when any of theoptical links from the network has failed, and communicating messages tothe OSNs in the adjacent nodes, as will be described in detail below.For the initial discussions, the OSN will be shown with a view todescribing the optical paths. The figure shows the OSNs in their default(normal) state where they act as direct connections between the networkand the ADMs. A detailed description of the OSN control electronics willbe set forth in a later section

As a matter of nomenclature, the term “node” is used in two contexts.First is in connection with the network topology, where the term node isused to signify a site where network transmissions may be initiated orterminated. Second is in connection with the optical switching node,which is a separate device that is placed at each node between theterminal equipment and the network fiber links. This should be clearfrom the context in which the term is used.

FIG. 6 also shows WDM couplers (multiplexers and demultiplexers) andadditional ADMs in association with each of OSNs 120 a and 120 b. Inparticular, optical signals between ADMs 50 a and 50 d and opticalsignals between ADMs 50 b and 50 c do not encounter WDM equipment whileoptical signals between ADMs 50 a and 50 b are optically multiplexed ordemultiplexed in connection with the other ADMs. For example, signals tobe sent from ADM 50 a onto working and protection fibers 55 abp′ and 55abw′ are optically multiplexed (combined) with signals from one or moreother ADMs 50 a′ by working and protection wavelength multiplexers 122aw and 122 ap. Similarly, signals for ADM 50 a coming in on working andprotection fibers 55 baw′ and 55 bap′ are optically demultiplexed (splitoff) from multi-wavelength signals on those fibers by working andprotection wavelength demultiplexers 123 aw and 123 ap. Similar WDMequipment is shown in association with ADM 50 b and one or more otherADMs 50 b′.

As a matter of terminology, “WDM terminal equipment” refers generally toWDM couplers and the like, while a “WDM terminal” typically refers to aparticular combination of WDM terminal equipment for multiplexing anddemultiplexing a particular set of fibers. In FIG. 6, working wavelengthmultiplexer 122 aw and working wavelength demultiplexer 123 awconstitute a WDM terminal, while protection wavelength multiplexer 122ap and protection wavelength demultiplexer 123 ap constitute another WDMterminal. WDM terminals typically include optical amplifiers andtransponders (optical-electrical-optical signal conversion units) foreach wavelength channel in addition to the multiplexer anddemultiplexer.

As noted above in connection with the discussion of FIG. 4, even thoughthe terminal equipment at either end of the multi-wavelength link ismulti-wavelength capable (i.e., has WDM terminal equipment andappropriately replicated SONET ADMs), the SONET ring is not capable ofrestoring multi-wavelength operation in the case of a failure in themulti-wavelength link. Rather, it is the OSNs, deployed and configuredaccording to embodiments of the invention, that provide such restorationcapability.

It should be noted that the invention is not limited to hybrid ringnetworks such as the one illustrated in FIG. 6. In FIG. 6, at least oneof the links carries multi-wavelength traffic and is terminated at bothends by WDM-equipped nodes; other links in the network carrysingle-wavelength traffic, and are terminated by nodes that areincapable of terminating multi-wavelength traffic. As mentioned above,such a hybrid network is one example of a broader class of heterogeneousnetworks where the communication regimes on some links differ in amaterial characteristic such as signal-carrying capacity (bandwidth),number of wavelength channels, modulation scheme, format, or protocol.Thus a heterogeneous network is characterized by a link, with terminalequipment at each end, where the signals on that link are incapable ofbeing transmitted on one or more other links, or are incapable of beingterminated by terminal equipment on one or more other links,or both.

The techniques of the present invention are in fact applicable to manytypes of heterogeneous ring networks. For example, different links coulddiffer in bandwidth due to different numbers of wavelengths supported byWDM terminal equipment at different nodes, even if all the links arecapable of supporting more than a single wavelength. Similarly,different links could differ in the bit rate or other electricalcharacteristics of the signals on a particular wavelength channel, evenif the different links had the same number of wavelength channels.

The considerations for upgrading the protection links in the moregeneral case are similar to the notion in the specific example of FIG. 6of upgrading single-wavelength links to support multi-wavelength trafficby providing optical amplifiers with a wider gain band. For example,protection links that normally carry traffic at a bit rate perwavelength channel that is lower than the highest bit rate in thenetwork might have to be upgraded by providing optical amplifiers withhigher gain or providing additional optical amplifiers to supporttraffic at a higher bit rate per wavelength channel. In some instances,it may be necessary to upgrade different protection links in differentways so that all of the protection links can carry the traffic that isnormally carried on all other working links. As will be discussed below,the invention does not require that the terminal equipment be upgradedto accommodate the “foreign” traffic. This is because the OSN has aswitching state that allows traffic on the protection fibers to bypassthe terminal equipment at the associated node.

2.2 OSN Overview

2.2.1 OSN Network Connections and Port Nomenclature

FIG. 7A is a schematic view showing additional details of OSN 120 a. ADM50 a′ is shown generally in the manner that ADM 50 a is shown in FIG.2B. The OSN includes west and east network ports, and west and eastterminal equipment ports. The figure shows the normal or defaultswitching configuration where no restoration is being undertaken. Inthis configuration, the OSN acts as a pass-through between the westnetwork ports and the west terminal equipment ports, and between theeast network ports and the east terminal equipment ports. This is onlyone of the many switching configurations for OSN 120 a, as will bedescribed in detail below. The solid lines indicate these defaultconnections.

Given that the right-hand (east) side of this particular ADM isconnected to a multi-wavelength link, the connection to the east side ofADM 50 a would be through WDM equipment as shown in FIG. 6. For clarity,the WDM equipment shown in FIG. 6 is omitted from FIG. 7A. As analternative view, each of the right-hand blocks designated Rx could bethought of conceptually as including a WDM demultiplexer and multipleopto-electronic receivers (e.g., photodiodes), each coupled torespective associated SONET ADM circuitry. Similarly, each right-handblock designated Tx could be thought of conceptually as including a WDMmultiplexer and multiple electro-optic transmitters (e.g., laserdiodes), each coupled to respective associated SONET ADM circuitry. Inthe particular example shown, the left-hand (west) side of ADM 50 awould not have associated WDM equipment.

As will be discussed below, during restoration due to failure of amulti-wavelength link, most of the OSNs in the ring have to provide abypass path for the multi-wavelength protection fibers. This is shown indashed straight lines passing from one side of the OSN to the other. Itis generally preferred to operate the OSN in connection with signalamplifiers (or regenerators) 125 a and 125 b, one for each of theprotection fiber bypass paths. To this end, the OSN further includesamplifier/regenerator ports for such connections. Regeneration willtypically be required if the nodes are separated by more than about 600km. The connections to the amplifiers (or regenerators) are shown ascurved dashed lines that cause the amplifier (or regenerator) to be partof the bypass path. Note that in this bypass path, the protection fiberis not in optical communication with the ADM.

FIG. 7B is a schematic view of OSN 120 a showing an alternativenomenclature for the OSN's network ports, terminal equipment ports, andamplifier/regenerator ports. The ports are shown as short arrowsindicating an input port or an output port (from the point of view ofthe OSN). FIG. 7B shows the ports in the same order and relationship tothe network and terminal equipment as FIG. 7B.

The OSN's input ports are designated as follows. Tx-W-West and Tx-W-Eastdesignate the working terminal ports coupled to the transmitters for thewest and east sides, while Tx-P-West and Tx-P-East designate theprotection terminal ports coupled to the transmitters for the west andeast sides. Similarly, W-West-(In) and W-East-(In) designate the workingnetwork ports for the west and east sides, while P-West-(In) andP-East-(In) designates the protection network ports for the west andeast sides. In a like manner, Regen-W-E-(In) and Regen-E-W-(In)designate the multi-wavelength signal ports coupled to the outputs ofregeneration (or amplification) equipment for signals traveling west toeast and east to west.

The OSN's output ports are similarly designated. Rx-W-West and Rx-W-Eastdesignate the working terminal ports coupled to the receivers for thewest and east sides, while Rx-P-West and Rx-P-East designate theprotection terminal ports coupled to the receivers for the west and eastsides. Similarly, W-West-(Out) and W-East-(Out) designate the workingnetwork ports for the west and east sides, while P-West-(Out) andP-East-(Out) designates the protection network port for the west andeast sides. In a like manner, Regen-W-E-(Out) and Regen-E-W-(Out)designate the multi-wavelength signal ports coupled to the inputs ofregeneration (or amplification) equipment for signals traveling west toeast and east to west.

2.2.2 OSN Switch Connections

FIGS. 8A and 8B are schematic views showing a number of possibleswitching paths within the OSN. FIG. 8A is drawn as an interconnectionmap between the inputs and the output ports of the OSN. Any number ofthese connections may be made exclusively or simultaneously in order toprovide the required operation of the optical switching node. The portnomenclature is as described above in connection with FIG. 7B, but theports are grouped by input and output ports.

FIG. 8B is drawn as a crosspoint matrix (grid) showing the circumstancesunder which different switch positions might be required. The OSN is asparse cross-bar in the sense that only a small fraction of the gridpositions are populated (22 out of 100).

3.0 OSN Details and Operation

3.1 OSN Switch States

3.1.1 Overview of OSN States

The following sequence of figures, including FIGS. 9A–9O, FIGS. 10A–10T,and FIGS. 11A–11R, show various states of the OSN required by variousconditions. The figures show the OSN as drawn in FIG. 7B, but with theports labeled only as protection or working. Each port whose fiber linkhas failed is shown with a round black arrowhead, the normal working andprotection connections are shown as solid lines, the restorationconnections are shown as heavy solid lines, and the original, but nolonger effective, connections are shown as dashed lines. The states havebeen grouped into four classes: (i) normal, (ii) span switching, (iii)ring switching, and (iv) protection fiber transit (P-transit). States inthe latter three classes are numbered and labeled by the protectionfiber or fibers that being used for restoration.

3.1.2 Span Switching States

FIGS. 9A–9O show 15 span switching states for the OSN. For ease ofreference, FIG. 9A also shows the OSN in its normal state. Spanswitching refers to a situation where a working fiber link has failedbut the corresponding protection fiber has not. In this context,reference to the corresponding protection fiber means the protectionfiber on the same side (east or west) and for use in the same direction(in or out with respect to the OSN). In this case, the terminal port (Rxor Tx) for the failed working fiber is coupled to the correspondingprotection fiber network port (in or out).

FIGS. 9A–9D show the span switching states for a single failed fiber.These states are designated as follows:

-   -   (ii) 1. Span Switch West-In;    -   (ii) 2. Span Switch West-Out;    -   (ii) 3. Span Switch East-In; and    -   (ii) 4. Span Switch East-Out.

FIGS. 9E–9J show the span switching states for two failed fibers. Thesestates are designated as follows:

-   -   (ii) 5. Span Switch West-In, West-Out;    -   (ii) 6. Span Switch East-In, East-Out;    -   (ii) 7. Span Switch West-In, East-Out;    -   (ii) 8. Span Switch West-Out, East-In;    -   (ii) 9. Span Switch West-In, East-In; and    -   (ii) 10. Span Switch West-Out, East-Out.

FIGS. 9K–9N show the span switching states for three failed fibers.These states are designated as follows:

-   -   (ii) 11. Span Switch West-In, West-Out, East-In;    -   (ii) 12. Span Switch West-In, West-Out, East-Out;    -   (ii) 13. Span Switch East-In, East-Out, West-In; and    -   (ii) 14. Span Switch East-In, East-Out, West-Out.

FIG. 9O shows the span switching states for four failed fibers. Thisstate is designated as follows:

-   -   (ii) 15. Span Switch West-In/Out, East-In/Out.

3.1.3 Ring Switching States

FIGS. 10A–10T show 20 ring switching states for the OSN. Ring switchingrefers to a situation where a working fiber and its correspondingprotection fiber have failed. Ring switching can occur in the absence orpresence of span switching. In this case, the terminal port (Rx or Tx)for the failed working fiber is coupled to the protection fiber networkport (in or out) on the other side. Accordingly, if the west working andprotection fibers have failed, the terminal port will be coupled to therelevant protection network port on the east side. As mentioned above,the labeling of the states refers to the side of the OSN (east or west)and direction (in or out) of the protection port that will couple to aviable protection fiber.

FIGS. 10A–10I show pure ring switching (working fiber and correspondingprotection fiber pair both fail). FIGS. 10A-10D show the ring switchingstates for one failed pair. These states are designated as follows:

-   -   (iii) 1. Ring Switch West-In    -   (iii) 2. Ring Switch West-Out    -   (iii) 3. Ring Switch East-Out    -   (iii) 4. Ring Switch East-In

FIGS. 10E and 10F show what is referred to as full ring switching,namely a circumstance where both pairs on one side have failed. Thesestates are designated as follows:

-   -   (iii) 5. Full Ring Switch West; and    -   (iii) 6. Full Ring Switch East.

FIGS. 10G and 10H show states where one pair on each side has failed.These states are designated as follows:

-   -   (iii) 7. Ring Switch West-In, East-Out; and    -   (iii) 8. Ring Switch West-Out, East-In. These two states are        actually not used in a current implementation since they would        not be useful for the particular type of terminal equipment.

FIGS. 10I–10T show states with simultaneous ring and span switching.These states arise where one pair on one side and one or two singleworking fibers have failed. These states are designated as follows:

-   -   (iii) 9. Ring Switch West-In, Span Switch East-Out;    -   (iii) 10. Ring Switch West-In, Span Switch West-Out;    -   (iii) 11. Ring Switch West-In, Span Switch East-Out, West-Out;    -   (iii) 12. Ring Switch West-Out, Span Switch East-In;    -   (iii) 13. Ring Switch West-Out, Span Switch West-In;    -   (iii) 14. Ring Switch West-Out, Span Switch East-In, West-In;    -   (iii) 15. Ring Switch East-In, Span Switch East-Out;    -   (iii) 16. Ring Switch East-In, Span Switch West-Out;    -   (iii) 17. Ring Switch East-In, Span Switch East-Out, West-Out;    -   (iii) 18. Ring Switch East-Out, Span Switch East-In;    -   (iii) 19. Ring Switch East-Out, Span Switch West-In; and    -   (iii) 20. Ring Switch East-Out, Span Switch East-In, West-In.

3.1.4 Protection Fiber Transit States

FIGS. 11A–11R show 18 protection fiber transit (P-transit) states forthe OSN. These states support the propagation of multi-wavelengthtraffic around the ring in the event of a failure of a multi-wavelengthlink in the ring. Since all the nodes are not guaranteed to have WDMterminal equipment, the switched traffic does not pass through any ofthe intervening terminal equipment, but rather only encounters the WDMterminal equipment on either side of the failed multi-wavelength link.

FIGS. 11A–11I show a first set of the transit states that do not use theamplifier/regenerator ports, but rather provide a straight through pathfrom an incoming protection fiber on one side od the OSN to the outgoingprotection fiber on the other side of the OSN. These states, whichinclude states where span switching is also occurring, are designated asfollows:

-   -   (iv) 1. P-Transit West-to-East;    -   (iv) 2. P-Transit West-to-East, Span Switch West-Out;    -   (iv) 3. P-Transit West-to-East, Span Switch East-In;    -   (iv) 4. P-Transit West-to-East, Span Switch West-Out, East-In;    -   (iv) 5. P-Transit East-to-West;    -   (iv) 6. P-Transit East-to-West, Span Switch West-In;    -   (iv) 7. P-Transit East-to-West, Span Switch East-Out;    -   (iv) 8. P-Transit East-to-West, Span Switch West-In, East-Out;        and    -   (iv) 9. P-Transit West-to-East and East-to-West.

FIGS. 11J–11R show a second set of the transit states that do use theamplifier/regenerator ports. These states correspond to the first set oftransit states except for the fact that the signals input on theprotection fiber are directed to the amplifier or regenerator beforebeing directed out on the protection fiber on the other side. Thesestates, which include states where span switching is also occurring, aredesignated as follows:

-   -   (iv) 10. P-Transit (Amp/Regen) West-to-East;    -   (iv) 11. P-Transit (Amp/Regen) West-to-East, Span Switch        West-Out;    -   (iv) 12. P-Transit (Amp/Regen) West-to-East, Span Switch        East-In;    -   (iv) 13. P-Transit (Amp/Regen) West-to-East, Span Switch        West-Out, East-In;    -   (iv) 14. P-Transit (Amp/Regen) East-to-West;    -   (iv) 15. P-Transit (Amp/Regen) East-to-West, Span Switch        West-In;    -   (iv) 16. P-Transit (Amp/Regen) East-to-West, Span Switch        East-Out;    -   (iv) 17. P-Transit (Amp/Regen) East-to-West, Span Switch        West-In, East-Out; and    -   (iv) 18. P-Transit (Amp/Regen) West-to-East and East-To-West.

3.2 OSN Detailed Implementation

FIG. 12 is a schematic showing a particular implementation of theoptical switching portion, referred to as optical switch block 150 (orsimply switch block 150), of an OSN having the states and functionalitydescribed above. From the interconnection map of FIG. 8A or thecrosspoint matrix of FIG. 8B, it can be deduced that all theconnectivity required in an OSN can be implemented by a small number ofswitches of 1×N and N×1 type. In particular, the connections shown canbe realized with two 1×3 switch elements, two 1×5 switch elements, two3×1 switch elements, and two 5×1 switch elements (8 switches). FIG. 12shows the switches and interconnections for switch block 150 explicitly.A particular state of the OSN can then be specified by the states of theswitches.

As can be seen, first and second 3×1 switches, designatedW_TERM_WEST_OUT_SW and W_TERM_EAST_OUT_SW have their single ou terminalscoupled to the west and east working receiver ports, while first andsecond 1×3 switches, designated W_TERM_WEST_IN_SW and W_TERM_EAST_IN_SWhave their single input terminals connected to the west and east workingtransmitter ports. Further, first and second 5×1 switches, designatedP_F₁₃ WEST_OUT_SW and P_F_EAST_OUT_SW, have their single outputterminals connected to the west and east network protection outputports, while first and second 1×5 switches, designated P_F_WEST_IN_SWand P_F_EAST_IN_SW, have their single input terminals connected to thewest and east network protection input ports. The multiple terminals onthe switches are connected to the other OSN ports or to multipleterminals on other switches to allow the OSN to assume the switch statesdescribed above.

While a general N×N crosspoint matrix switch may be used to implementthe required functions of an OSN, an implementation such as that shownin FIG. 12 provides significant economies. A full 10×10 switch matrixwould require 10 1×10 switch elements and 10 10×1 switch elements.Further, in many switch technologies, 1×3 and 1×5 switch elements arefar easier and cheaper to fabricate than 1×10 or 10×1. Thus thepreferred implementation of the OSN offers savings in the number ofswitches (8 versus 20) as well as the cost per switch. Even though thespecific OSN uses a sparse 10×10 matrix (see FIG. 8B), OSNs for variousother network configurations can be designed using the same approach,possibly with a different number of ports or a different desired set ofstates.

The OSN can use a variety of switch technologies. These include, but arenot limited to semiconductor optical amplifier based switch elements andoptical directional couplers (1×N and N×1), electro-optic and polymerbased lightwave switches (1×N and N×1), opto-mechanical lightwaveswitches (1×N and N×1), and integrated lightwave circuits to realize theoptical switching node. In a current implementation, opto-mechanicalswitches procured from E-TEK Dynamics, Inc. of San Jose, Calif. wereused. Optical switches are generally reversible (at least for passiveswitch technologies), so whether a switch is a 1×N switch or an N×1switch depends on the way it is connected.

3.3 OSN Controls and Software

FIG. 13 is an optical and electrical schematic of an embodiment of OSN120 a, and shows additional details of the OSN, including control logicand additional elements to implement a restoration supervisory channel.As mentioned above, the OSN control circuitry is used to operate theoptical switching node and provide the necessary messages which aretransmitted over an optical restoration supervisory channel to adjacentnodes. The messages sent on the restoration supervisory channel aresometimes referred to as pilot tones. The communication medium for thesupervisory messages is the fiber network itself, and the messages aremerged with the network data using WDM couplers, as will now bedescribed.

The elements in OSN 120 a include, in addition to switch block 150, WDMcouplers for placing messages on the network links and taking them offthe links. Each input network port has an associated WDM demultiplexer160, which directs the optical network signals to the correspondinginput network port on switch block 150 and directs the opticalsupervisory messages to a respective opto-electrical receiver 165. Thesignals from receivers 165 are directed to control logic 170. Similarly,each output network port has an associated WDM multiplexer 180, whichcombines the optical network signals from the corresponding outputnetwork port on switch block 150 with optical supervisory messagesgenerated by a respective opto-electrical transmitter 185. Transmitters185 are driven electrically by control logic 170. Control logic 170communicates with a circuit 190, which controls and drives the opticalswitches in switch block 150.

Data processing circuits for ATM or other data processing are associatedwith the transmitters and receivers. Conceptually, they can beconsidered part of the transmitters and receivers, or part of thecontrol logic. In a specific implementation, PMC 5346 S/UNI Lite chipsare used.

The supervisory messages are carried on a wavelength that is removedfrom the wavelengths of the network data messages (1530–1570 nm), andtypically are at a lower bit rate (say OC-3 or 155 Mbps). Thus, whilethe fiber amplifiers in the network may not provide as much gain as theydo for the signals in the main wavelength band, the detectors inreceivers 165 do not need as much gain for the signals at the lower bitrate. Candidate wavelengths include 1310 nm, 1480 nm, 1510 nm, and 1625nm, with 1510 nm being presently preferred.

A computer such as an embedded processor 200 is coupled to the controllogic, and stores restoration software in an on-board or off-chipnon-volatile memory (e.g., PROM or flash EPROM). The restorationsoftware: (a) processes incoming supervisory messages and makes logicaldecisions for operation of the optical switches (i.e., to set theswitches to the appropriate state); and (b) generates supervisorymessages to be sent to adjacent nodes to allow them to set theirrespective states accordingly.

While it is possible to implement centralized control of the OSNs in thenetwork, it is preferred to have each node operate autonomously on thebasis of signals it receives from its adjacent nodes. Each OSN sends“keep alive” messages to its adjacent nodes at regular intervals, andeach OSN monitors such incoming messages to detect a loss of signal.Depending on which fiber link has failed, OSN processor 200 determineswhich type of switching needs to be performed, and operates to controlthe switches accordingly. The OSN also sends messages to its adjacentnodes, informing them of the action taken, so they can reconfigurethemselves accordingly.

3.4 OSN Operation

FIG. 14 illustrates the operation of the OSNs and OSN software in onepossible scenario, namely a complete failure of all four fiber linksbetween OSNs 120 a and 120 b. As shown in the figure, the protectionfibers have been switched to provide WDM signal transit while leavingthe working fibers between the ADMs undisturbed. The ultimate state ofthe ring would have the OSNs in the following states:

-   -   OSN 120 a in state (iii) 5. Full Ring Switch West (FIG. 10E);    -   OSN 120 b in state (iii) 6. Full Ring Switch East (FIG. 10F);        and    -   OSNs 120 c and 120 d in state (iv) 9. P-Transit West-to-East and        East-to-West (FIG. 11I).        However, no single message from any of the OSNs would cause        this, but rather a sequence of messages would be required, as        will now be described. Assume that no other abnormal conditions        were existing at the time of the failure.

In this case OSN 120 a would detect the loss of incoming signals on itseast side and send a message in both directions that it has detectedsuch a loss. OSN 120 a would then set the appropriate switches to routesignals incoming on P-West-(In) to Rx-W-East.

Meantime, OSN 120 b would detect the loss of incoming signals on itswest side and send a message in both directions that it has detectedsuch a loss. OSN 120 b would then set the appropriate switches to routesignals incoming on P-East-(In) to Rx-W-West.

In response to the messages from OSNs 120 a and 120 b, OSNs 120 c and120 d would determine that full ring switching was to be in effect, andwould set their appropriate switches to route signals incoming onP-West-(In) to P-East-(Out) and signals incoming on P-East-(In) toP-West-(Out).

In the configuration shown in FIG. 14, OSNs 120 c and 120 d transitionto their respective pass-through transit states without regeneration. Ingeneral, it is not necessary that every OSN have an associatedregenerator. Rather, as noted above, optical/electrical/opticalregeneration is only required at 600-km intervals, and so it may be thatonly some of the OSNs in the network have associated regenerators. Tothe extent that a given OSN has an associated regenerator, it wouldenter state (iv) 18. P-Transit (Amp/Regen) West-to-East and East-To-West(FIG. 11R).

3.5 SONET (ADM) Switching and OSN Switching

While FIG. 14 shows the restoration in ring 110 using the OSNs, it doesnot address the issue of how restoration occurs in the othersingle-wavelength rings that share the multi-wavelength link but may nothave OSNs. These other rings do their normal ring switching ascontrolled by the ADMs on those rings. When the multi-wavelength link isrestored through the OSN switching as described above, these ADMsrecover traffic on their original working ports and revert to normaloperation.

The SONET switching and the OSN switching can operate withoutinterfering with each other. For example, there is no constraint on therelative speed of ring switching response time. Put another way, the OSNswitching time does not need to be faster than the SONET ring switchingtime (5–50 ms). Thus, if the SONET switching (in the ADMs) occurs fasterthan OSN restoration, the ADMs revert to their original state after OSNswitching. On the other hand, if the SONET switching occurs more slowlythan the OSN restoration, the ADMs do not see a (verified) break on thelink before traffic is restored through the OSNs. Thus, a racingcondition is never created since the switching processes are mutuallyindependent.

3.6 Restoration Software Details

FIGS. 15A–15C, taken together, form a flowchart of the OSN software in aspecific embodiment. As described above, each OSN has two inputs(working and protection) on each side (West and East). Thus it canmonitor the presence or absence of signal on these two inputs. Whenthere is any disruption in the signal, the software resident at the OSNdetermines which signal failed, and changes state of the switches to asuitable position such that the lost signal can now be received from aprotection fiber input, either from the same direction (span switching)or the opposite direction (ring switching). While the switches are beingchanged to the new state, the OSN also communicates this change of stateand any action request/instruction to its adjacent OSNs if necessary sothat the adjacent OSNs can take appropriate action to route the signals.

Each OSN communicates with its two adjacent nodes over all four fibersinterconnecting the OSNs. In the current implementation, ATM packets areused over each communication channel to send and receive messages. Eachsuch channel between the OSNs is referred to as a restorationsupervisory channel. The restoration messages sent by each OSN containlocal information about that node including:

-   -   Node i.d. (a unique i.d. assigned to each node);    -   Logical state of the node;    -   Physical state of the switches at the node;    -   Status of any equipment faults at the node (e.g., failure of        laser or other hardware)        Each OSN also sends instructions to other nodes for carrying out        certain actions given the local knowledge at that node.

The OSN software is described in further detail in the followingmicrofiche appendices filed as part of U.S. patent application Sir.No.08/408,002, now U.S. Pat. No.6,331,906, filed Sept.29, 1999, of RohitSharma and Larry R. McAdams, entitled “METHOD AND APPARATUS FOROPERATION, PROTECTION AND RESTORATION OF HETEROGENEOUS OPTICALCOMMUNICATION NETWORK ” and incorporated by reference in their entiretyfor all purposes:

-   -   Appendix 1—96 pages of source code on;    -   Appendix 2—(51 pages) Span Switch Restoration States (file        Restoration_States_Span) contains pseudo-code for the        Span-Switch states (ii) .x;    -   Appendix 3—(62 pages) Ring Switch Restoration States (file        Restoration_States_Ring) contains pseudo-code for the        Ring-Switch states (iii) .x;    -   appendix 4—(58 pages) P-Transit Switch Restoration States (filed        Restoration —States —Transit) contains pseudo -code for Transit        States (iv) .x;    -   Appendix 5—(52 pages) Adjacent Node Action Request Table (file        ANAR_Table) sets forth the logical states to which nodes are to        transfer (and the corresponding messages to be sent) on receipt        of certain messages from the adjacent nodes. Each node has a        WEST and an EAST adjacent node. The tables specify what the        software is to do when an adjacent node requests a certain        action.        4.0 Additional Features and Alternatives

4.1 OSN Sub-Module

FIG. 16A shows how a protection WDM terminal 205 at a network node canbe used in place of a separate regenerator. The ADM (one of multipleADMs at this node) and OSN at this node are designated by respectivereference numbers 50 and 120, corresponding to earlier figures. Thispossible elimination of the need for a separate regenerator is based ona recognition that when the OSN at the node is in one of its bypassstates, the relevant portions of the ADM's protection circuitry are notin use and the protection ports of the ADMs can be bypassed. The figureshows the protection paths into the WDM and the ADM with the bypass pathfor one of the wavelength channels drawn as a heavy line. The workingfiber paths are omitted for clarity.

A set of separate switching arrays, referred to as OSN sub-modules 210are disposed between the WDM terminal and the ADMs. Each sub-moduleincludes as many 1×2 switches as there are wavelength channels. FIG. 16Ashows how regeneration can be provided for west-to-east transit usingthe WDM terminal that interfaces the ADMs to the link on the west sideof the node. It will be apparent that if the node has WDM equipment thatinterfaces the ADMs to the link on the east side of the node, additionalsub-modules can be disposed between the additional WDM equipment and theeast sides of the ADMs at the node.

FIG. 16B shows the OSN sub-modules in their normal state, where theycouple the protection fibers to the ADM protection ports as if thesub-modules were not there. FIG. 16C shows the OSN sub-modules in theirADM bypass state where they bypass the ADM and cause the WDM terminal toact as a pass-through regenerator. The portion of the WDM terminal forthe protection fiber from the OSN includes an optical amplifier 215, awavelength demultiplexer 220, and separate transponders 222 for eachwavelength channel. Similarly, the portion of the WDM terminal for theprotection fiber to the OSN includes separate transponders 227 for eachwavelength channel, a wavelength multiplexer 230, and an opticalamplifier 235.

The regeneration (signal conditioning) takes place in the transpondersthat are part of the protection WDM terminal. Thus, the incomingwavelength channels on the single fiber are first optically amplified byamplifier 215 and optically demultiplexed by demultiplexer 220 ontoseparate fibers, whereupon the individual wavelength channels areconverted to electrical signals, which are conditioned and reconvertedto optical signals by transponders 222. In the ADM bypass state, the OSNsub-module then routes the individual optical signals to the otherportion of the WDM terminal where the signals are conditioned bytransponders 227 and then put on a suitable wavelength for multiplexingby multiplexer 230, amplification by amplifier 235, and transmissionthrough the fiber. It will be appreciated that the bypass path couldpossibly include a separate WDM terminal depending on how the WDMterminals are deployed in the node.

4.2 Bidirectional Supervisory Channel and Network Channel

As described above and shown in FIG. 13, supervisory messages are senton working and protection fibers only in the direction of the networkdata traffic on those fibers. It is possible, however, and there arepotential benefits to having bidirectional supervisory messages sent oneach fiber, even if that fiber is only carrying network data in onedirection. In the system with unidirectional supervisory messages, theOSN only “learns” of a failure in an outgoing link when the OSN at theother end of the link fails to receive messages and notifies theremaining OSNs of that fact. Thus the message regarding the failureneeds to propagate around the ring, which can slow down the restorationswitching.

FIGS. 17A and 17B show two implementation options for realizingbidirectional supervisory channel message transmission. These arefragmentary views corresponding to portions of FIG. 13. For a givennetwork fiber (incoming or outgoing with respect to the OSN), the singlereceiver (165 in FIG. 13) for an incoming fiber or the singletransmitter (185 in FIG. 13) for an outgoing fiber is replaced by areceiver 165′ and a transmitter 185′.

FIG. 17A shows an implementation where the incoming and outgoingsupervisory messages are on two different wavelength channels, which areremoved from the wavelength channel or channels dedicated to the networkdata. A bidirectional WDM coupler 240 serves both as a multiplexer anddemultiplexer for the supervisory wavelength channels. FIG. 17B shows animplementation where the incoming and outgoing supervisory messages areon the same wavelength channel. Separation is achieved using a broadbandoptical coupler 242 and an isolator 245. In both these views, thenetwork data is shown as being on wavelength channels numbered 1 to n,but this discussion applies equally to single-wavelength andmulti-wavelength data links (i.e., n could be 1).

FIG. 18 shows bidirectional WDM transmission on one of the networkfibers. As alluded to above, while the specific embodiments usedseparate fibers for each direction, it is possible to providecommunication in both directions on a single fiber for the networktraffic. This is true for single-wavelength or multi-wavelength networkdata transmission in each direction. As in the case of the bidirectionalsupervisory channels shown in FIGS. 17A and 17B, bidirectional WDMtransmission can be implemented using a bidirectional WDM coupler 250.The figure also shows the supervisory channel devices shown in FIG. 13,namely demultiplexer 160, receiver 165, multiplexer 180 and transmitter185.

Bidirectional operation allows inbound and outbound working orprotection traffic to be multiplexed onto one fiber using separatewavelength bands. Specifically, WDM coupler 250 operates as ademultiplexer to direct incoming optical signals on a network fiber 252onto a fiber 255 while multiplexing outgoing optical signals on a fiber260 onto network fiber 252. For generality, the figure shows n outgoingwavelength channels and m incoming wavelength channels, but either orboth of m and n could be 1.

4.3 Other Terminal Equipment

FIGS. 19A-19C show OSN 120 deployed in networks having other types ofterminal equipment such as ATM and IP. Reference numbers correspondingto those in FIG. 16A will be used where appropriate.

FIG. 19A shows OSN 120 coupled to ADM 50 as well as an ATM switch 270and an IP switch or router 275 through protection WDM terminals 205 andworking WDM terminals 205″ and OSN sub-modules 210. The figure alsoshows secondary data equipment 280 (typically IP) coupled to the OSNthrough the OSN sub-modules and the protection WDM terminal.

The OSN sub-modules shown in the figure can serve several functions. Forsimplicity, the bypass path is not shown, but the OSN sub-modules in theprotection paths can be switched to cause protection WDM terminals 205to act as regenerators for transit states of the OSN as described abovein connection with FIGS. 16A–16C. In the normal state of these OSNsub-modules, as shown, the protection channels are coupled to thesecondary data equipment so that the protection fibers on the networkcan be used during times when they are not needed for restoration. TheOSN sub-modules in the working paths are not needed, but may be deployedto provide additional versatility. It should be understood that thesecondary data equipment would not have access to the network duringrestoration.

FIG. 19B shows an OSN at a node where there are no SONET ADMs, butrather only ATM equipment 270 and IP equipment 275 coupled to thenetwork working links through working WDM terminals 205′ and the OSN.Also shown are secondary IP equipment 282 coupled to the networkprotection links through protection WDM terminals 205 and the OSN.

FIG. 19C shows a variant of the configuration in FIG. 19B where the ATMand IP equipment on the west side have secondary data ports coupledthrough the protection WDM terminal and OSN to the west side networklink in the manner that secondary data equipment and 280 in FIG. 19A,secondary IP equipment 282 in FIG. 19B, and secondary IP equipmentcoupled to the east side in FIG. 19C.

5.0 Conclusion

In conclusion it can be seen that the present invention providespowerful and elegant techniques for providing enhanced restoration inoptical fiber networks. Full protection of multi-wavelength links in ahybrid network is achieved without having to provide WDM terminalequipment at nodes between single-wavelength links in the network.Desired switching can be effected using relatively simple andinexpensive optical switching nodes, typically using only a small numberof n×1 and 1×n switches where n is less than the number of inputs andoutputs of the node.

While the above is a complete description of specific embodiments of theinvention, various modifications, alternative constructions, andequivalents may be used. For example, FIG. 13 shows the OSN and itscontrol electronics as being responsible for implementing thesupervisory channel by generating and monitoring messages from adjacentnodes. It is possible, however, to have the terminal equipment implementthe supervisory channel for controlling the OSN. Since WDM terminalsystems monitor the signals on a link, a WDM-terminal controlled versionof the OSN can also be implemented. Such an OSN would be completelycontrolled by the WDM terminals, which would determine the need for spanswitching or ring switching through the use of the WDM supervisorychannel and other monitoring features. The physical configuration of theOSN would be similar to the particular implementation described above.The WDM terminal control system (Element Manager) would be required tosend the required control signals (to set the OSN in one of the validstates for restoration). Such a configuration would have lower loss(since WDM couplers for the supervisory channel would not be required)and lower complexity since less processor capability would be required.

Therefore, the above description should not be taken as limiting thescope of the invention as defined by the claims.

1. An optical switching device, the optical switching device beingoptically coupled to first and second bi-directional network links,comprising: a set of first network ports for optically connecting theoptical switching device to a first bi-directional network link, whereinthe set of first network ports includes first bi-directional working andprotection network ports, which optically connect the optical switchingdevice to the first bi-directional network link; a set of second networkports for optically connecting the optical switching device to a secondbi-directional network link, wherein the set of second network portsincludes second bi-directional working and protection network ports,which optically connect the optical switching device to the secondbi-directional network link; wherein each of the first and secondbi-directional network links comprises incoming working and protectionlinks and outgoing working and protection links; a set of first terminalequipment ports for optically connecting the optical switching device toa first portion of terminal equipment; a set of second terminalequipment ports for optically connecting the optical switching device toa second portion of the terminal equipment; switching elements connectedto the sets of first and second network ports and the sets of first andsecond terminal equipment ports, the switching elements performingswitching for an optical network that utilizes bi-directional networklinks that carry both multi-wavelength and non-multi-wavelength signals,wherein the performed switching includes at least one of span, ring, andprotection transit switching, and wherein the performed switching doesnot interfere with SONET layer switching of the signals; wherein theterminal equipment is configured to send and receive data from the firstand second bi-directional network links via first and second networkinterfaces, respectively, wherein the first network interface isconfigured to be optically connected to the first bi-directional workingand protection ports and the second network interface is configured tobe optically connected to the second bi-directional working andprotection ports; and a control device operable to control the opticalswitching device based on at least one of detected failures andsupervisory messages.
 2. An optical network utilizing the opticalswitching device of claim 1, wherein the optical network comprises aplurality of N nodes (N>2) configured in a ring topology, each of thenodes being optically connected to first and second adjacent nodes viathe first and second bi-directional network links, respectively.
 3. Theoptical network of claim 2, wherein: the first bi-directional networklink corresponding to each node includes the first bi-directionalworking and protection links optically connected to the first adjacentnode, the second bi-directional network link corresponding to each nodeincludes the second bi-directional working and protection linksoptically connected to the second adjacent node, and each node processessignals carried over one or more wavelengths in the working links of thecorresponding first and second bi-directional network links.
 4. Theoptical network of claim 3, wherein at least one node includes theoptical switching device for optically connecting the at least one nodeto the corresponding first and second bi-directional network links, theoptical switching device being configured such that: the set of firstnetwork ports includes first bi-directional working and protectionnetwork ports, which optically connect the at least one node to thecorresponding first bi-directional working and protection links,respectively; and the set of second network ports includes secondbi-directional working and protection network ports, which opticallyconnect the at least one node to the corresponding second bi-directionalworking and protection links, respectively.
 5. The optical network ofclaim 4, wherein the optical switching device performs protectiontransit switching by optically connecting the first and secondbi-directional protection network ports to a bypass path that bypassesterminal equipment of the at least one node, thereby opticallyconnecting the corresponding first and second bi-directional protectionlinks via the bypass path.
 6. The optical network of claim 5, wherein:the first bi-directional working and protection links are eachconfigured to carry signals in a clockwise and counter-clockwisedirection with respect to the ring topology, and the secondbi-directional working and protection links are each configured to carrysignals in the clockwise and counter-clockwise direction.
 7. The opticalnetwork of claim 6, wherein: the first bi-directional protection networkport includes clockwise and counter-clockwise ports corresponding to theclockwise and counter-clockwise directions, respectively, the secondbi-directional protection network port includes clockwise andcounter-clockwise ports corresponding to the clockwise andcounter-clockwise directions, respectively, and the optical switchingdevice performs the protection transit switching by switching at leastone of the clockwise and counter-clockwise ports for each of first andsecond bi-directional protection network ports from being opticallyconnected to first and second terminal equipment ports, respectively, tobeing optically connected to the bypass path.
 8. The optical network ofclaim 6, wherein the first bi-directional working and protection linksand the second bi-directional working and protection links arecollectively implemented in two optical fibers.
 9. The optical networkof claim 8, wherein the two optical fibers are each configured to carrysignals in only one direction with respect to the ring topology.
 10. Theoptical network of claim 8, wherein the two optical fibers are eachconfigured to carry signals in a bi-directional manner with respect tothe ring topology.
 11. The optical network of claim 8, wherein at leastone of the two optical fibers of each bi-directional network link isconfigured to carry multi-wavelength signals.
 12. The optical switchingdevice of claim 1, wherein the terminal equipment is configured toterminate a second type of communications carried over the first andsecond bi-directional network links, the second type of communicationscorresponding to a second number of wavelength channels which is lessthan the first number of wavelength channels.
 13. An optical switchingnode including the optical switching device of claim 1, the opticalswitching node being implemented in an optical network.
 14. The opticalswitching node of claim 13, wherein: the optical network includes aplurality of N terminal equipment nodes (N>2) configured in a ringtopology, such that each of the terminal equipment nodes are opticallyconnected to first and second adjacent terminal equipment nodes via thefirst and second bi-directional network links, respectively, and each ofthe terminal equipment nodes includes terminal equipment configured toterminate a first type of communications carried over the first andsecond bi-directional network links.
 15. The optical switching node ofclaim 14, wherein the optical switching device is configured such that:the set of first network ports includes the first bi-directional workingand protection network ports, which optically connect the opticalswitching node to the first bi-directional network link, and the set ofsecond network ports includes the second bi-directional working andprotection network ports, which optically connect the optical switchingnode to the second bi-directional network link.
 16. The opticalswitching node of claim 15, wherein the optical switching deviceperforms protection transit switching by optically connecting the firstand second bi-directional protection network ports to a bypass path thatbypasses the terminal equipment of a terminal equipment node, therebyoptically connecting the first and second bi-directional protectionlinks via the bypass path.
 17. The optical switching node of claim 16,wherein at least two of the terminal equipment nodes in the opticalnetwork have terminal equipment configured to terminate a second type ofcommunication carried over the first and second bi-directional networklinks, and at least one of the terminal equipment nodes does not includeterminal equipment configured to terminate the second type ofcommunication.
 18. The optical switching node of claim 17, wherein thefirst type of communication corresponds to carrying single-wavelengthsignals, and the second type of communications corresponds to carryingmulti-wavelength signals.
 19. The optical switching node of claim 18,wherein each bi-directional network link includes at least two opticalfibers.
 20. The optical switching node of claim 19, wherein at least oneof the two optical fibers in each bi-directional network link isconfigured for the second type of communication.
 21. The opticalswitching device of claim 1, wherein the supervisory messages arecarried on signals having one or more wavelengths, which are distinctfrom wavelengths of signals carrying network data messages.
 22. Theoptical switching device of claim 1, wherein the control device isconfigured to: detect a failure by detecting a loss of incoming signalson at least one of the first and second bi-directional network links,determine a type of switching to be performed by the optical switchingdevice based on the detected failure, and generate supervisory messagesbased on the detected failure to be sent on the first and secondbi-directional network links.
 23. The optical switching device of claim1, wherein the control device is configured to receive a supervisorymessage on at least one of the first and second bi-directional networklinks, and process the received supervisory message to determine a typeof switching to be performed for the optical network.
 24. The opticalswitching device of claim 1, wherein the control device controls theoptical switching device to perform span switching when there is adetected failure in the incoming working link, but not the incomingprotection link, of at least one of the first and second bi-directionalnetwork links, thereby causing incoming signals to be received over theincoming protection link of the at least one of the first and secondbi-directional network links.
 25. The optical switching device of claim1, wherein the control device controls the optical switching device toperform span switching when there is a detected failure in the outgoingworking link, but not the outgoing protection link, of at least one ofthe first and second bi-directional network links, thereby causingoutgoing signals to be sent over the outgoing protection link of the atleast one of the first and second bi-directional network links.
 26. Theoptical network switching device of claim 1, wherein the control devicecontrols the optical switching device to perform ring switching whenthere is a detected failure in both the incoming working and protectionlinks of one of the first and second bi-directional network links,thereby causing incoming signals to be received over both the incomingworking and protection links of the other of the first and secondbi-directional network links.
 27. The optical switching device of claim1, wherein the control device controls the optical switching device toperform ring switching when there is a detected failure in both theoutgoing working and protection links of one of the first and secondbi-directional network links, thereby causing outgoing signals to besent over both the outgoing working and protection links of the other ofthe first and second bi-directional network links.
 28. The opticalnetwork switching device of claim 1, wherein: the control device causesthe optical switch to perform protection transit switching when theincoming protection link of at least one of the first and second bi-directional network links is configured to carry multi-wavelengthsignals and the terminal equipment is not configured to processmulti-wavelength signals, and the performed protection transit switchingcauses the protection link of the incoming protection link of the atleast one of the first and second bi-directional network links to beoptically connected to the outgoing protection link of the other of thefirst and second bi-directional links while bypassing the terminalequipment.
 29. The optical network switching device of claim 1, whereinthe terminal equipment includes: at least one add/drop multiplexerconfigured to originate and terminate signals carried over the first andsecond bi-directional network links.
 30. The optical network switchingdevice of claim 29, wherein the at least one add/drop multiplexer is aSONET add/drop multiplexer.
 31. The optical switching device of claim29, wherein the terminal equipment includes: at least one wavelengthdivision multiplexing (WDM) coupler disposed between the at least oneadd/drop multiplexer and one of: the first bi-directional workingterminal equipment port, the first bi-directional protection terminalequipment port, the second bi-directional protection working terminalequipment port, and the second bi-directional protection terminalequipment port.
 32. The optical switching device of claim 31, whereinthe terminal equipment includes: at least one optical switchingsub-module disposed between the at least one add/drop multiplexer andthe at least one WDM coupler, wherein the at least one WDM coupler andthe at least one optical switching sub-module is configured toregenerate a signal carried over the first and second bi-directionalnetwork links when the optical switching sub-module is controlled tobypass the at least one add/drop multiplexer.
 33. The optical switchingdevice of claim 1, wherein the terminal equipment is configured toterminate a first type of communications carried over the first andsecond bi-directional network links, the first type of communicationscorresponding to a first number of wavelength channels. device of claim1, the optical switching node being implemented in an optical network.34. A method of utilizing an optical switching device to configure anoptical network, the optical network including a plurality of N nodes(N>2) configured in a ring topology, the method comprising: providing anoptical switching device, providing the optical switching devicecomprising: providing a set of first network ports for opticallyconnecting the optical switching device to a first bi-directionalnetwork link, wherein the set of first network ports includes firstbi-directional working and protection network ports, which opticallyconnect the optical switching device to the first bi-directional networklink; providing a set of second network ports for optically connectingthe optical switching device to a second bi-directional network link,wherein the set of second network Ports includes second bi-directionalworking and protection network ports, which optically connect theoptical switching device to the second bi-directional network link;wherein each of the first and second bi-directional network linkscomprises incoming working and protection links and outgoing working andprotection links; providing a set of first terminal equipment ports foroptically connecting the optical switching device to a first portion ofterminal equipment; providing a set of second terminal equipment portsfor optically connecting the optical switching device to a secondportion of the terminal equipment; and providing switching elementsconnected to the sets of first and second network ports and the sets offirst and second terminal equipment ports, the switching elementsperforming switching for an optical network that utilizes bi-directionalnetwork links that carry both multi-wavelength and non-multi-wavelengthsignals, wherein the performed switching includes at least one of span,ring, and protection transit switching, and wherein the performedswitching does not interfere with SONET layer switching of the signals;wherein the terminal equipment is configured to send and receive datafrom the first and second bi-directional network links via first andsecond network interfaces respectively, wherein the first networkinterface is configured to be optically connected to the firstbi-directional working and protection ports and the second networkinterface is configured to he optically connected to the secondbi-directional working and protection ports; providing a control deviceoperable to control the optical switching device based on at least oneof detected failures and supervisory messages; providing each of thenodes with the first and second bi-directional network links thatoptically connect the node to first and second adjacent nodes; andimplementing the optical switching device in at least one node.
 35. Themethod of claim 34, wherein the providing each of the nodes with firstand second bi-directional network links step further comprises:configuring the first bi-directional network link to include the firstbi-directional working and protection links, which are opticallyconnected to the first adjacent node; and configuring the secondbi-directional network link to include the second bi-directional workingand protection links, which are optically connected to the firstadjacent node, wherein each node processes signals carried over one ormore wavelengths in the working links of the corresponding first andsecond bi-directional network links.
 36. The method of claim 35, whereinthe implementing step further comprises: configuring the set of firstnetwork ports to include the first bi-directional working and protectionnetwork ports, which optically connect the at least one node to thecorresponding first bi-directional working and protection links,respectively; and configuring the set of second network ports to includethe second bi-directional working and protection network ports, whichoptically connect the at least one node to the corresponding secondbi-directional working and protection links, respectively.
 37. Themethod of claim 36, further comprising: using the optical switchingdevice to perform protection transit switching by optically connectingthe first and second bi-directional protection network ports to a bypasspath that bypasses the terminal equipment of the at least one node,thereby optically connecting the corresponding first and secondbi-directional protection links via the bypass path.
 38. The method ofclaim 37, wherein the providing each of the nodes with first and secondbi-directional network links step further comprises: configuring each ofthe first bi-directional working and protection links to carry signalsin a clockwise and counter-clockwise direction with respect to the ringtopology; and configuring each of the second bi-directional working andprotection links to carry signals in the clockwise and counter-clockwisedirection.
 39. The method of claim 38, wherein: the first bi-directionalprotection network port includes clockwise and counter-clockwise portscorresponding to the clockwise and counter-clockwise directions,respectively, the second bi-directional protection network port includesclockwise and counter-clockwise ports corresponding to the clockwise andcounter-clockwise directions, respectively, and the using step performsthe protection transit switching by switching at least one of theclockwise and counter-clockwise ports for each of the first and secondbi-directional network ports from being optically connected to first andsecond terminal equipment ports, respectively, to being opticallyconnected to the bypass path.
 40. The method of claim 38, furthercomprising: implementing the first bi-directional working and protectionlinks and the second bi-directional working and protection links in twooptical fibers.
 41. The method of claim 40, further comprising:configuring the two optical fibers to carry signals in only onedirection with respect to the ring topology.
 42. The method of claim 40,further comprising: configuring the two optical fibers to carry signalsin a bi-directional manner with respect to the ring topology.
 43. Themethod of claim 40, further comprising: configuring at least one of thetwo optical fibers in each bi-directional network link to carrymulti-wavelength signals.